As GPS systems become more accurate, GPS is used for more location related applications. For example, motorists now commonly use GPS systems as navigation aids. Vehicle fleets, such as taxis, school buses or police and fire may be tracked using GPS. Surveyors may also use GPS as a tool of the trade. Location systems like GPS may be used in more mission critical applications as the accuracy of the system improves.
In essence, a GPS unit determines its current position by determining a distance between the GPS unit and GPS satellites that transmit signals. This difference is typically determined through either code phase resolution or carrier phase resolution. Code phase resolution involves measuring the distance between a GPS receiver and a transmitting GPS satellite by using a transmitted code. Determining a position through code phase resolution usually results in position accuracy measured in meters.
Carrier phase measurement may be used when greater GPS accuracy is needed. Carrier phase measurement involves measuring the distance between a GPS receiver and a transmitting GPS satellite by using a transmitted carrier frequency. Errors in position calculation occur, however, when there is a discontinuity in the measured carrier phase. This discontinuity may result from a temporary loss in the carrier tracking loop of a GPS receiver, satellite blockage, jamming, clock noise, or for other reasons. The loss of carrier may result in a temporary loss of location information. This temporary loss of location information is not acceptable in some critical applications.
One way of providing accurate location information, which eliminates the problem of temporary carrier loss, is to use an inertial navigation system (INS). Inertial navigation systems typically measure change in position through mechanical devices such as accelerometers, gyroscopes or other motion-sensing devices. One advantage of INS is that it requires no external references in order to determine its position. A disadvantage is that error accumulates in an IMS system as time passes. An INS system may be minutely inaccurate over a very short time period, but the longer INS is used to determine position the less accurate the system becomes. For critical applications, a system that exhibits temporary loss of position or a system that accumulates error over time does not provide sufficiently accurate position information.